Transformer



' 1,519 258 Dec. 19246 w. G. HETTICH TRANSFORMER Filed Jan. 22. 1920 Patented Dec. 16, 1924.

UNITED STATES 1,519,258 PATENT OFFICE.

WILLIAM G. HETTICH, OF CHICAGO, ILLINOIS, ASSIGNOR TO STANDARD XJ-RAY QOM- PANY, OF CHICAGO, ILLINOIS, ADOBPORATION OF ILLINOIS.

TRANSFORMER Application and January 22, 1920. Serial No. 353,321.

1 '0 all whom may concern:

Be it known that I, WILLIAM G. Harmon,

a citizen of the United States, residing f at Chicago, in the county of Cook and State of Illinois, have invented certain new and useful Improvements in Transformers,of which the following is a specification.

This invention relates to transformers.

It will be explained as applied to a stepup transformer especially adapted for use with X-ray tubes.

X-rayltubes are operated by high .voltage current from a step-up transformer. The high voltage alternating current from the transformer is sometimes rectified before application to the X-ray tube and sometimes it is not.

An X-ray tube ordinarily is required to perform two widely differing classes of serv- 108., One class of service necessitates relatively heav'y current delivery to the tube.

for relatively short periods of time; the otherclass of service ordinarily requires only relatively small current delivery to the tube for relatively long periods of time.

For the heavy current short time service, such for exam le as some classes of roentgenography, t e dan er to attendant and patient, from the big voltage heavy current circuits, is relatively small because of the extremely short time of application. The opportunities for inadvertent approach to or contact with the hi h voltage circuits are greatly lessened as t e time of application shortens.

For the relatively small current prolonged time service, such for example as some classes of treatments and direct examinations, the danger to the attendant and patient is relatively great if heavy current will be delivered by the transformer in the event that the resistance of its secondary circuit is materially reduced. The opportunities for inadvertent approach to or contact with the high voltage circuits, resulting in reduction of secondary circuit resistance, are greatly increased as the time of application is lengthened. Heretofore more or less protection during small current prolonged time service has been provided by ballast resistance and circuit breakers in the primar circuits of the transformers. However bot of these expedients areunsatisfactory, inefficie-nt and unreliable. Ballast resistance has been foundinefiicient because of the energy used up or wasted in the rheostat. Ballast resistance has further been found unsatisfactory because the resistanoe changes as the temperature chan cs. Circuit breakers have been found unreliable because they do not always operate, and because of the time intervalrequired for them to act.

One of the objects of this invention is to provide greater protection in the operation of X-ray tubes.

Another object is to provide a transformer which may be used for both heavy current and small current service, and will afford adequate protection during the small current service.

Another object is to provide a transformer from which heavy secondary current may be taken when necessary and in which the secondary current is automatically limited to relatively small values when small currents only are required. 3

Another object is to provide a transformer from which a relatively small secondary crrrent may be taken at a voltage which remains practically constant regardless of the length of time the transformer is used.

Another object is to provide a transformer which provides protection at a lower cost than the cost of a machine made with an ordinary transformer and with a protective rheostat or circuit breaker.

Another object is to provide a transformer which is simple and reliable and obviates the necessity for extraneous protective apparatus.

Other objects and advantages will hereinafter appear.

An embodiment of the invention is illustrated in the accompanying drawing which shows one form of transformer in section and diagrammatic circuit connections.

In accordance with this invention the transformer has a single high voltage secondary winding and two low voltage primary windings wound on a common magnetic core.

One primary winding, for convenience called the heavy dut primary, is so related to the secondary an the common core that the leakage of the flux induced by the current in the heavy duty primary varies but little over a wide range of secondary current. In other words the regulation with the heavy duty rlmary is relatively close or good over a wi e range of secondary current. The

other primary winding, for convenience called the li ht duty primary, is so related to the secondary and the common. core that the leaka e of the flux induced by the current in the -light duty primary increases greatly as the secondary current rises above relatively a small amount. In'other words the regulation with the light duty primary is relatively poor except for a narrow range of small secondary current.

Thus, when the heavy duty primary is used the secondary current may be greatly increased. On the other hand when the light duty primary is used the value to which the secondary current may rise may be limited to a very small amount.

These characteristics may be obtained by using a rectangular closed iron core on which the secondary winding encircles the heavy duty primary winding which is wound, for example on two opposite legs of the core. The heavy duty primary is preferably relatively long and is distributed evenly over that-portion of the core which. is encircled by the seconda so that its leakage path is of relatively igh reluctance. The light duty primary, onthe other hand, should be concentrated and relatively short or otherwise designed to have a leakage path of relatively low reluctance, and placed on ontof the other sides of the core, preferably as far removed as possible from the secondary.

The counter magneto-motive force or flux induced by current in the secondary opposes the magneto-motive force or flux induced by current in theprimaries. When the heavy duty primary'is used the counter ma etomotive' force induced by the current in the secondary causes but little "of the flux induced by the primary to leave the core and become leakage flux. This condition obtains over a wide .range of seconda current values because the relatively hig reluctance leakage path keeps the greater part of the flux in the corq where it is effective to induce voltage in the secondary. Therefore, with the heavy duty primaryv the magnetic linkage or coupling between the primary and secondary remains relatively close and the 'ratio of transformation remains substantially constant over a wide range of secondary current values. As the secondary circuit resistance decreases, with the resultant tendency of the secondary current to increase, the primary current will correspondingly increase to induce such greater secondary current. Thus the regulation with the heavy duty primary is relatively good throughout a comparatively wide range of secondary circuit resistance.

When the light duty primaryis used, and the secondary current is relatively small, i. e., the secondary circuit resistance is relatively high, the counter magneto-motive force induced by the secondary causes but little of the flux induced by the primary to leave is driven from the core and becomes leakage flux and the windings become relative y loosely magneticall linked or coupled. The

leakage flux is ine ective to induce voltage in the secondary, and the secondary current will not rise pro rtionally to the decrease in resistance of t e secondary circuit. Furthermore the characteristic high self induction of a concentrated winding contributes to limit the maximum value to which the light duty primary current may rise. Therefore instead of the secondary current continuin to increase as the resistance of the secon ary circuit decreases, which is the condition obtaining when 'the heavy duty primary is used, the light duty primary may provide an-increase in secondary current over a relatively small decrease in secondary.resis'tance, after which the secondary current can increase relatively little upon further lowering of the secondary circuit resistance. Thus the regulation with the light duty primary may be good over a relatively narrow range of secondalyhxsurrent, but becomes relatively poor...as. .narrow range is-ex V The transformer illustratedin the drawing has a closed rectan gular iron core 1. 'A

heavy duty primary windi com' rising two coils 2 and 3, conn toget er in series by a conductor 4, is wound u n opposite limbs 5 and 6, respectively, 0 core 1. Coils 2 and 3 are relatively long so that the leakage paths of each, that is the paths outside of the core, are of relatively high reluctance.

The secondary winding com rises two coils 7 and 8, wound over the eavy duty primary coils 2 and 3, respectively. The secondary coils may be made up of series of sections separated by insulation 9 in the usual manner and may be connected in series with each other by a conductor 10. The terminals of the secondary winding are respectively connected by conductors 11 and 12 to the X-ray apparatus in the usual manner.

The secondary winding is thus superposed upon the heavy duty primary and magnetically closely linked or coupled thereto. This arrangement of the heavy duty rimary and secondary windings together with the relatively high reluctance leakage paths of the heavy duty primary provide a substantially constant ratio of transformation between the primary and secondary over a wide range of secondary circuit resistance variation. This condition is due to the fact that the effective magnetic flux; i. e.: the flux influencing the secondary to induce voltage therein, remains about equal to the flux produced by the heavy duty primary. In other words there is relatively little leakage of flux over a relatively wide range of secondary circuit resistance or current.

A light duty primarywinding 13 is placed upon one or more of the other limbs of the core. This primary winding is relatively short and loosely linked magnetically with the secondary. 'Its leakage path is relatively of low reluctance compared to the reluctance of the leakage path of the heavy duty primary.- Consequently when the light duty primary is used. the counter magnetomotive force produced by current in the sec ondary and acting against the flux produced current in the primary is able, at relatively low secondary current values, to drive part of the total flux of the primary out of that part of the core which underlies the secondary, and thus increase the leakage. This leakage flux is relatively ineffective on the secondary so that, as the leakage increases, the ratio of transformation between the light duty primary and the secondary falls. The leakage flux does not link with the secondary windings and'is therefore ineffective to induce voltage'therein, thus causing the secondary voltage to decrease and preventing the'secondary current from increasing proportionately to the decrease in resistance of the secondary circuit. The most of this leakage flux leaves the core near one end of the light duty rimary winding and enters the core near t re other end of that winding in muchthe same manner that flux travels through-the air around the winding of a choke coil consisting of a. straight iron core with a winding encircling the middle port-ion thereof. This choke coil duty primary contributes to limit the value to whlch the primary current and the magneto-motive force induced thereby may rise when the resistance of the secondary circuit 13 ly reduced. The output of the secon ary winding is thus limited by two factors, when the light duty rimary winding is used: First, the secondary voltage decreases as the secondary current and the leakage flux increase, and secondly, the choke coil action ofthe small capacity primary assists in limiting the magneto-motive force available to produce the flux which induces voltage in the secondary. Until the counter magneto-motive force is able to drive out lines of force created by the light duty primary against the relatively low reluctance of the leaka e path, the leakage will be relatively small, the magnetic coupling will be relatively close and the ratio of action of the light transformation will remain relatively constant \Vhen, however, the counter-magnetomotive force of the secondary is able to appreciably increase the primary fiux flowing over the relatively low reluctance leakage paths the magnetic coup-ling becomes relatively loose and the ratio of transformation falls. Consequently as the secondary circuit resistance decreases, the current increase proportionally over a small range; but. when the secondary resistance drop exceeds a relatively small amount, the current no longer increases proportionately to the resistance drop. The amount of current which may be taken from the secondary without any excessive drop in seconda voltage is governed by the relutance of the leakagev path of the light duty primary windings.

The terminals of the heavy duty. and light duty primary windings may be connected to a source of alternating current 14, through suitable regulating devices, in the usual manner. One terminal of each Winding is connected by conductors 15, 16 andl'l to one terminal of the source. The other terminal of the heavy duty primary is connected to a switch contact 18 by a conductor 19. Similarly the other terminal of the light duty primarypis connected to a switch contact 20 by a conductor 21. Contacts 18 and 20 may be alternately connected to the other terminal of source 13 by a switch blade 22.

V hen switch blade 22 is moved to engage contact 18 the transformer is connected for heavy current short time service. The resistance of the secondary circuit may be reduced over a wide range to provide the necemry current for heavy duty service.

When switch blade 22 is moved to engage contact 20, the transformer is connected for small current long time service Over a relatively small range of secondary circuit resistance drop the secondary current will proportionally increase. However, when this relatively small range of resistance drop and resultant current increase'js exceeded, for example when a patient or attendant should touch or dangerously approach the high voltage circuits, the counter magnetomotive force induced by the current in the secondary causes the leakage of magneto flux to increase rapidly and the current rise in the secondary will be relatively small. The secondary voltage will fall rapidly al.- most to zero if the secondary current much exceeds the low value which the transformer is designed to deliver.

Thus with one transformer the common secondary may be caused to possess two rad ically different characteristics of output. One in which the secondary current may be inc ased over a wide range and the other in hich the secondary current can not be increased beyond a relatively small amount.

Having described my invention what I claim as new and desire tosecure by Letters Patent, is

1. A transformer having a secondary winding, and a pair of primary windings associated therewith, one primary being associated with the secondary so that the ratio between effective and leakage fluxes remains substantially constant over a wide variation in secondary circuit resistance and the other primary being associated with the secondary so that the ratio between effective and leakage fluxes varies materially over a relatively narrow range of secondary circuit resistance.

2. A transformer having a core with an effective magnetic circuit of low reluctance,

a secondary winding thereon, a heavy duty primary winding on the core and having a relatively high reluctance leakage circuit, and a light duty primary winding on the core and having a leakage circuit of relatively low reluctance.

3. A transformer closed core of magnetic material, a secondary winding wound over a portion of the core, a heavy duty primary wound over the same portion of the core as the secondary so that the flux induced in the core by the primary is largely effective on the secondary over a wide range of secondary resistance, and a light duty primary wound on a different portion of the core so that, below a relatively narrow range of secondary resistance, the flux induced by the primary is having a substantially largely a leakage flux and ineffective on the secondary.

4. A step-up transformer having a sub stantially closed core of magnetic material, a common secondary winding wound over a portion of the core, a heavy duty primary winding wound upon the core in superposed relation to the secondary and having relat-ively high reluctance leakage paths so that over a wide range of secondary circuit resistance the leakage is relatively constant, and a. light duty primary winding wound on a different part of the core and having relativelylow reluctance leakage paths so that below a relatively narrow range of secondary circuit resistance the leakage becomes relatively great to limit the current and voltage output of the secondary.

5. A transformer having a closed magnetic core, a secondary winding on the core, a heavy duty primary winding on the core and relatively closely magnetically linked to the secondary winding under all normal ranges of secondary circuit resistance, and a light duty primary winding which is relatively closely magnetically coupled to the secondary winding until the secondary circ uit resistance decreases through a relatively narrow range and becomes relatively loosely magnetically coupled to the secondary winding when the secondary circuit resistance decreases below the range.

In testimony whereof I hereunto subscribe my name.

WILLIAM G. HETTICH. 

